diagnostic shows P0420,how do i repair that.

1997 MERCURY MARQUIS
90,000 MILES
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DEBBIE TAUFI
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  • 1 POST
catalyst system Efficiency Below Threshold Bank 1 is what shows up. Took it in auto zone did a diagnostic and showed cadalack coverter. but today showed the P0420
Jun 8, 2012 at 4:36 PM
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HMAC300
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  • 48,601 POSTS
  • ASE Certified Mechanic
best to have a pro look at it. it can be a converter bad, a coolant leak cooling off the converter, a bad O2 sensor, many things.
Jun 8, 2012 at 4:48 PM
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